The present invention relates to a method for the distribution of drive torque to the wheels of a driven axle of a motor vehicle.
A differential or balancing gear serves for compensating different rotational movements of wheels of a motor vehicle. Such a differential is therefore provided on each driven axle of a motor vehicle, in order, particularly during cornering, to compensate different rotational speeds of the bend-inside (corner-inside) and of the bend-outside (corner-outside) wheel. Differentials which are assigned to a driven axle are conventionally designed as what are known as open differentials. They have the function of a torque balance and generally provide a torque equilibrium between the left and the right driving wheel. If there are road conditions with different coefficients of friction on the driving wheels (“μ-split”), then the transmittable propulsion force of the vehicle is limited on account of the balance effect to the double value of the propulsion force of the wheel with the lower coefficient of friction. If there is an excessive drive torque, this wheel then spins.
Furthermore, it is known (Bosch—Kraftfahrtechlisches Taschenbuch [Motor Vehicle Manual], 24th edition, Vieweg Verlag, page 668) to avoid or to reduce the abovementioned undesirable effect by the positive or nonpositive locking of the differential. Positive locks are switched on by the driver. In light of the distortion of (tension in) the drive train which in this case occurs, positive locks, as a rule, are switched on only in the case of all-wheel vehicles driving off-road.
Of the known nonpositive locking differentials there are those with a fixed degree of locking, torque-sensing locking differentials (for example, Torsen differential), and locking differentials sensing rotational speed (for example, Visco clutches). These differentials are all passive locking differentials.
Electronically controlled locking differentials (what are known as active locking differentials), in which the degree of locking is controlled by a drive dynamics controller in the vehicle, are also known.
Passive locking differentials can influence the driving behavior during cornering positively, but also negatively. In principle, in cornering free of any transverse force, a locking differential generates an understeer torque on account of its locking function. This understeer torque is based on the distortion (generated by the locking function) between the more slowly running bend-inside wheel and the faster running bend-outside wheel and on the coupling of the two wheels via the road. The distortion torque acts positively on the bend-inside wheel and negatively on the bend-outside wheel and consequently generates the understeer torque. In cornering with low transverse accelerations, this behavior is considered to be negative.
When the vehicle negotiates a bend (i.e. drives a corner) with higher transverse acceleration, the bend-inside wheel is relieved and the bend-outside wheel is loaded. With the bend-inside wheel being increasingly relieved, the distortion torque can no longer be fully supported, since the adhesion potential of the bend-inside wheel is reduced. Similarly to this, the drive force on the bend-outside wheel rises. This force then generates a yawing moment which supports turning the vehicle into the bend. Understeering is reduced, and, as compared with a vehicle with an open differential, a higher transverse acceleration can be achieved, since the adhesion potential of the bend-outside wheel can be utilized more effectively.
In electronically controlled differentials, it has been known hitherto to increase the degree of locking essentially as a function of traction. Active locking differentials of this type are therefore used predominantly in off-road vehicles. However, electronic locking differentials are also used in order to improve handling in sports vehicles (for example, F430).
The document DE 197 33 674 A1 discloses a method for increasing the driving stability of a motor vehicle, the engine drive torque being increased on a bend and at the same one of the driven wheels being braked in order thereby to regulate to a desired yawing moment.
It is known from DE 199 54 131 A1 to calculate a virtual bend radius from wheel rotational speeds, to calculate a desired radius from the steer angle and, as a function of these, to activate an assigned brake or differential lock of a differential gear.
Furthermore, it is known from the document DE 196 37 193 A1 to provide a special differential which sets up different ratios between the driven wheels.
DE 10 2005 018 069 A1 discloses a control for active wheel steering, and from the document DE 196 01 795 it is known to increase the yawing moment by braking the bend-inside rear wheel.
Furthermore, DE 102 36 734 A1 shows a steering actuator, by means of which, in addition to or instead of the setting of a steer angle, a longitudinal force can be applied to at last one vehicle wheel in order to follow a desired bend path.
Finally, the laid-open publication DE 198 13 736 A1 discloses a control system for regulating the driving stability, specifically by the selective braking of individual wheels, wherein understeering is to be prevented.
The document DE 10 2004 046 008 A1 discloses a drive train in which the drive torque is distributed to the driven wheels of an axle by means of what is known as a twin clutch. The twin clutch has two individual clutches, the input members of which are connected to one another and receive drive torque. The output members are in each case connected to one of the driven wheels. Such a twin clutch allows directed torque distribution and, moreover, replaces a conventional transverse differential.
Furthermore, the document U.S. Pat. No. 6,120,407 discloses a driven axle which has a conventional open transverse differential. Furthermore, each drive shaft is assigned a planet wheel set arrangement and an activatable clutch. The wheel set arrangements in each case act between the differential cage and the drive shaft and make it possible, as required (for example, in cornering), to set up a rotational speed difference between the wheels.